A xe2x80x9cdigital subscriber loopxe2x80x9d (xe2x80x9cDSLxe2x80x9d) is a type of communications connection and/or service which is now being offered by many local exchange carriers (e.g., telephone companies) to consumers and businesses alike as a way of receiving faster Internet connections and downloads.
Digital, multi-tone transceivers (xe2x80x9cDMTsxe2x80x9d) are devices which are designed to send and receive DSL-compatible signals (hereinafter xe2x80x9cDSL signalsxe2x80x9d).
The quality of a DSL signal received by a DMT depends on a number of factors. One such factor is the amount of interblock interference (xe2x80x9cIBIxe2x80x9d). In an attempt to avoid the undesirable effects of IBI, DMTs are designed to generate and insert a guard time sequence, called a cyclic prefix, between each data block. In order to completely eliminate IBI, the cycle prefix has to be at least as long as the impulse response of the channel through which DMTs transmit DSL signals. However, in practice this can rarely be achieved, necessitating the use of supplemental methods and devices to minimize IBI. One such device is a time domain equalizer (xe2x80x9cTEQxe2x80x9d).
Generally speaking, a TEQ is an electronic filter which filters out the effects of IBI. More specifically, a TEQ is a type of xe2x80x9cfinite impulse responsexe2x80x9d (xe2x80x9cFIRxe2x80x9d) filter.
FIG. 1 depicts a graph of a typical, simplified impulse response, h(n), of a DSL channel. Traditionally, the graph shown in FIG. 1 is described as having a xe2x80x9cnon-causalxe2x80x9d portion, (xe2x80x9cNCxe2x80x9d), and a xe2x80x9ccausalxe2x80x9d portion, (xe2x80x9cCxe2x80x9d). The non-causal section comprises, what will be referred to as a xe2x80x9clower tail,xe2x80x9d while the causal portion comprises xe2x80x9cmain lobexe2x80x9d and xe2x80x9cupper tail sections,xe2x80x9d respectively.
Existing TEQs have been able to reduce IBI by reducing the power in the tails. However, existing TEQs have not been able to substantially minimize IBI.
FIG. 2 depicts a graph of an xe2x80x9cidealxe2x80x9d, theoretical impulse response where the power within both tails has been reduced to zero. Simplified, eliminating the power within both tails means eliminating the tails altogether.
Comparing FIG. 1 with FIG. 2, it can be seen that the signal levels of points 1-4 have been lowered to zero. Whereas the impulse response in FIG. 1 contained power in each of its tails (represented by the area under each tail, PL and Pu, respectively), such power has been eliminated within the ideal impulse response shown in FIG. 2.
FIG. 3 depicts a simplified block diagram of a circuit 100 comprising a TEQ 104. It should be understood that this xe2x80x9ccircuitxe2x80x9d may comprise integrated circuits, discrete devices, or the like.
As shown, a signal S1 is transmitted by transmitter 101 through a communications channel 102. The channel 102 can be characterized by an impulse response 102a. Signal S2 represents a distorted version of signal S1. Some of the distortion is due to IBI. Before being received by a receiver 103, the signal S2 is input into a TEQ 104 in order to reduce IBI. It should be noted that transmitter 101 and receiver 103 may both comprise DMT/DSL transceivers or the like and that the TEQ 104 is typically a part of receiver 103.
In an attempt to minimize IBI, existing TEQ""s utilize xe2x80x9cequalizer coefficientsxe2x80x9d (hereafter xe2x80x9ccoefficientsxe2x80x9d) which are adapted to reduce the power under the tails of a composite impulse response (i.e., combination of the TEQ and original channel). This approach reduces IBI, but it does not substantially minimize IBI.
To make the explanation which follows as clear as possible, these coefficients can be thought of as xe2x80x9cweights.xe2x80x9d That is, a TEQ is adapted to filter the impulse response with its weights so as to lower the power in the tails of the composite impulse response. There exists methods and devices which utilize such weights to so reduce IBI. Again, though IBI is reduced, it is not substantially minimized.
A major reason why IBI is not minimized is due to a fundamental flaw regarding the nature of how different points along a tail contribute to IBI.
To date, it has been assumed that different points along the tails of an impulse response contribute equally to IBI. Based on this erroneous assumption, existing TEQs are designed so that the coefficients uniformly reduce the power in each tail.
The present inventor discovered that each portion of a tail, for example, portions represented by points 1-4 in FIG. 1, contribute non-uniform amounts of power to IBI.
Accordingly, it is a desire of the present invention to provide for methods and devices which substantially minimize IBI in DMTs.
It is another desire of the invention to provide for methods and devices which substantially minimize IBI in DMTs by taking into consideration the fact that each portion of a tail of a channel""s impulse response contributes a non-uniform amount of IBI.
Other desires will become apparent to those skilled in the art from the following description taken in conjunction with the accompanying drawings and claims.
In accordance with the present invention there are provided methods and devices for substantially minimizing interblock interference. One device envisioned by the present invention comprises a finite impulse response filter adapted to apply differential coefficients to a channel""s impulse response. Such a device may comprise a DMT, or DSL transceiver to name just a few examples.
The differential coefficients envisioned by the present invention are derived from a novel weighting matrix of differential factors. The factors are selected by taking into account that each point along a tail of a channel""s impulse response contributes a different amount of IBI. More specifically, points located the furthest from the time of reference of a channel""s impulse response contribute more IBI than points located closer (e.g., point 1 in FIG. 1 contributes more IBI than point 2).
Novel filters making use of such differential coefficients are applied to tails of the impulse response to substantially minimize IBI.
Both devices and methods are envisioned by the present invention.